The present invention relates to lasers, and, in particular, relates to interacting with laser beams.
A laser emits a beam of light of a unique nature which is of a fixed wavelength, for example. The use of this beam in a particular application may require a different wavelength. This may be accomplished either intracavity or external thereto with the use of, for example, second-harmonic generators. The use of nonlinear crystals for frequency conversion processes is well known. The use of high power laser beams therein can severely degrade the nonlinear crystals because of heating due to the absorption of light. Thermal gradients lead to gradients in the indices of refraction which disrupt phase matching and can adversely affect the transverse beam quality. Excessive heating may also lead to crystal damage. Optical beam scanning provides a means to spread the thermal load over the cross-section of the crystal, while preserving the small beam cross-section, and consequent high peak intensity needed for efficient frequency conversion. The scanner must keep the beam direction constant in the crystal to preserve phase matching. The scanner must further both introduce a transverse scanning motion of a few millimeters to the beam incident on the crystal and remove this motion from the beam after it leaves the crystal. The scanner must also not alter the optical polarization relative to the crystal since only particular polarization states are phase matched. Rapid scan rates may be desirable to correspond to convenient submultiples of optical pulse frequencies.
The use of beam scanning to produce second harmonic generators is described, for example, in Laser Handbook, vol. 3, pp. 421-484, 1979, which is incorporated by reference. In one arrangement, five lithium niobate crystals were placed in an oven and rode on a vibrating rail at 4 Hz between two springs which resulted in intracavity doubling of Nd:YAG radiation. The device clearly would not be suitable for a high scan rate and would be difficult to implement with a critically phase-matched interaction because of narrow angular acceptance. Other devices may use a scanning mirror having a saw-tooth motion or a rotating annular ring of a uniaxial crystal. Both of these devices impart a net beam deflection and are thus unsuitable for insertion into a laser beam cavity, and further the later may be affected by the dependence of the effective nonlinear coefficient on azimuthal angle.
Thus there exists a need for a device which causes a laser beam to interact with a frequency conversion device that is able to handle high powers and not alter the outgoing beam direction.